It is common to use a heat dissipation device, one type being commonly known as a heat sink, which is placed in contact with the back surface of an electrical device such as a semiconductor for effectuating a cooling function to prevent an over temperature condition of the device when operating at high power levels. The device is attached to the heat sink usually using a multiplicity of fasteners which require very accurate machining and a high degree of assembly labor especially when a large number of devices are sequentially mounted.
Heat conduction is improved with the use of a solder or a heat conductive paste applied to the mounting side of a thermal pad which is a part of the semiconductor device and is usually bolted to a heat sink through a hole in the flange of the thermal pad. The thermal pad is attached to the device active elements and functions as a heat conductive plate whose purpose is to conduct the heat generated by the elements and is directly attached to a larger more effective heat dissipator, such as a heat sink, which requires a large amount of space on the printed circuit board.
It is also known to use a cooling jacket to cool the devices where the devices themselves are immersed in a cooling fluid or the devices are attached and in thermal contact with a heat sink which contains one or more cooling channels wherein a cooling fluid is forcibly circulated.
Problems have arisen when the electrical device, such as a power semiconductor, is mounted to a circuit board for cooling by heat transferred through the heat conductive thermal pad and where electrical connection leads that emanate from the device are attached to connection pads, known as solder pads, on the printed circuit board. To obtain adequate cooling, an auxiliary heat sink must be added in some manner to effectively conduct additional heat from the thermal pad of the electrical device to control the operating temperature. It is known to mount the semiconductor directly to a heat sink having fins which extend into the surrounding air and are mounted to the printed circuit board, however, this approach is not conducive to high packaging density on the printed circuit board itself since the heat sink requires a large area which results in a larger overall size of the electronics module. It is thus desirable to provide a method of mounting the heat sink such that it can effectively conduct heat away from the semiconductor device while allowing for dense packaging of the electrical components on the printed circuit board with a minimum number of fasteners and a relaxation of the geometric tolerances involved.
The stringent geometric requirements and accuracy of the prior art methods and high labor content are not conducive to high volume manufacturing assembly of the printed circuit board where the heat sink is mounted directly to the semiconductor device increasing cost especially where a multiplicity of devices are mounted sequentially. Also, the prior art methods do not generate sufficient clamping loads on the electrical device to establish a good thermal conductivity between the heat conductive thermal pad and the heat sink. It is desirable to have a large contact area between the semiconductor thermal pad and the heat sink to maximize the heat transfer and the cooling effect. Prior art methods, such as that described in U.S. Pat. No. 4,479,140, U.S. Pat. No. 5,089,936, and copending U.S. application U.S. Ser. No. 07/894,678 filed on Jun. 5, 1992 assigned to the same assignee as the present application, the disclosures of which are hereby incorporated by reference, describe methods to use springs to assure contact between a semiconductor device and the heat sink.
These methods are effective but require additional parts, such as springs, mounting bolts and a retention block. Prior art devices do not allow for the semiconductor device to be mounted directly on the circuit board so that the electrical leads can be soldered to the printed circuit board soldering pads to minimize induced stresses either at the solder pads or along the electrical leads themselves while allowing for high clamping loads to maximize the heat conduction and into the heat sink. Also, the heat sink is mounted on the same side of the circuit board as the semiconductor device which does not allow for optimum location of the electrical components for dense packaging and complicates the assembly process.